The invention relates to a method for preparing chemically bonded cementitious materials, particularly phosphate-bonded calcium aluminate cements.
Non-hydraulic, magnesium phosphate cements have been described in U.S. Pat. No. 4,436,555 to Sugama et al. These magnesium phosphate cements (MPC) are derived by mixing magnesium oxide as a bivalent cation-releasable solid and aqueous ammonium polyphosphate (Poly-N) as a cement-forming solution. The chemical reaction between the MgO solid and Poly-N liquid generates high levels of activation energy resulting in rapid setting. Subsequent heating of the cement results in anhydrous magnesium orthophosphate. A shortcoming of this MPC material is the susceptibility of nonreactive MgO remaining in the cement to hydrolysis leading to expansion, decomposition and reduction in strength.
It has now been discovered that the utilization of calcium aluminate cement as a solid reactant in phosphate-bonded cementitious materials avoids such hydrolytic deterioration. With this cement, the species of bivalent cation reacting with aqueous ammonium polyphosphate is calcium.
Calcium aluminate cements have been manufactured, for example, by heating until molten or sintering a mixture of limestone and bauxite with a small amount of SiO.sub.2 FeO and TiO.sub.2. These cements are extensively used for refractory concretes. Calcium aluminate cement is commercially available as, for example, Lumnite and Refcon, both supplied by the Lehigh Cement Co. of Allentown, Pa.
Calcium aluminate cements have commonly been used in hydraulic cements which usually contain lime, alumina and silica hardened with a mixture of water. For example, U.S. Pat. No. 4,915,740 to Sakai et al. describes a hydraulic cement which contains as a main ingredient, calcium alumina cement containing ultra-fine particles, a metal aggregate, an organic acid plasticizer and a phosphoric acid ester as a set adjusting agent. A molded article may be cured in high pressure in water and dry cured.
U.S. Pat. No. 4,683,003 to Nakano et al. discloses production of cellular concrete from a mixture of an aqueous slurry of calcium silicate, an aqueous slurry of calcium aluminate, with a hydroxycarboxylic acid setting retarder in both slurries, and an aqueous foaming agent.
Portland cement is another much-used hydraulic cement made by mixing alumina and calcium oxide and burning. Various formulations for Portland cement are described, for example, in U.S. Pat. Nos. 4,230,499; 4,187,118; 3,885,979; 3,717,488 and 3,677,780. Aluminous, i.e. high alumina, cements contain at least 30-35% alumina, in contrast to Portland cement which contains less than 5% alumina. Aluminous cement attains its maximum strength more rapidly than Portland cement and is more resistant to solutions of sulfates.
In other advances using alumina and calcium oxides or hydroxides, U.S. Pat. No. 4,670,055 to Koslowski describes a method for controlling quick setting of a mixture of alumina cement and calcium hydroxide by premixing the aluminate with an organic acid and calcium sulfate. U.S Pat. No. 4,076,545 to Azuma et al. describes calcium aluminate monosulfate hydrate building materials made by mixing calcium oxide, alumina, calcium sulfate and water at 100.degree.-200.degree. C. and under pressure. There is no phosphate bonding in these preparations.
Metal oxides such as FeO have been known to increase strength in building components. U.S. Pat. No. 4,126,475 to Hansen et al. discloses a steam-cured compressed mixture of laterite (aluminum oxide, 4-50 weight percent iron oxide, and silicon oxide) lime (CaO) and water. Reacted iron oxide forms supplementary binding in the product. U.S. Pat. No. 4,459,156 to Henslee et al describes a mortar composition of MgAl.sub.2 O.sub.4 spinels and aluminum phosphate or polyphosphates. U.S. Pat. No. 2,391,493 to Wainer et al. describes casting compositions of an inert material such as sand, acid phosphate, magnesia containing iron oxide and a fluoride.
Chemically bonded ceramics in the Ca-SiO.sub.2 -P.sub.2 O.sub.5 -H.sub.2 O (hydroxy apatite) quaternary system have been prepared using calcium nitrate, lactate, phosphoric acid and silica, hydrated at 85.degree. C.-300.degree. C., as reported Hu, J., et al., "Studies of Strength Mechanism in the Newly Developed Chemically Bonded Ceramics in the System CaO-SiO.sub.2 -P.sub.2 O.sub.5 -H.sub.2 O", Cement and Concrete Research 18, 103-108 (1988) and Hu et al., "Investigation of Hydration Phases in the System CaO-SiO.sub.2 -P.sub.2 O.sub.5 -H.sub.2 O", Journal of Materials Research 3, 772 (1988). The preparation of hydroxy apatite from Ca.sub.2 P.sub.2 O.sub.7 ; and CaO under hydrothermal conditions has been described by T. Hattori et al., "Hydrothermal Preparation of Calcium Hydroxyapatite Powders" J Am. Chem. Soc. 73(6) 1803-805 (1990). An article by R. A. Steinke, et al., "Reactions and Bonding of Sol-Gel Derived Chemically Bonded Ceramics in the System CaO-P.sub.2 O.sub.2 -SiO.sub.2 ", Advanced Cements and Chemically Bonded Cements, MRS Int'l On Adv. Mats., 13, 229 (1989), also reports hydroxyapatites formed by sol-gel preparation of compositions of calcium nitrate, silica and phosphoric acid. Pressed powders were cured at 35.degree. C. and 90.degree. C. under pressure. These articles describing hydroxyapatite do not suggest the use of calcium aluminate cement and their compositions do not attain the high strengths of the present invention.
Therefore, it is an object of the present invention to utilize interaction processes and reaction products occurring between ammonium polyphosphate and calcium aluminate cement.
It is another object of the invention to provide rapid-setting material which can be used for concrete bridgedecks, highways and airport runways and for their rapid repair.
It is a further object to provide phosphate-bonded cement which avoids expansion, decomposition and reduction in strength resulting from hydrolysis of unreacted metal oxides within the cement.
It is yet another object to provide an economical cementitious material with high compression strength.
It is still another object to provide an economical cementitious material using readily available commercial materials.
It is an object of the invention to provide phosphate-bonded calcium aluminate cements.